Millimeter Wave Thin-Film Bulk Acoustic Resonator in Sputtered Scandium Aluminum Nitride

This work reports a millimeter wave (mmWave) thin-film bulk acoustic resonator (FBAR) in sputtered scandium aluminum nitride (ScAlN). This paper identifies challenges of frequency scaling sputtered ScAlN into mmWave and proposes a stack and new fabrication procedure with a sputtered Sc0.3 Al0.7 N on Al on Si carrier wafer. The resonator achieves electromechanical coupling ( ${k} ^{2}$ ) of 7.0% and quality factor ( ${Q}$ ) of 62 for the first-order symmetric (S1) mode at 21.4 GHz, along with ${k} ^{2}$ of 4.0% and ${Q}$ of 19 for the third-order symmetric (S3) mode at 55.4 GHz, showing higher figures of merit (FoM, ${k} ^{2} \cdot {Q}$ ) than reported AlN/ScAlN-based mmWave acoustic resonators. The ScAlN quality is identified by transmission electron microscopy (TEM) and X-ray diffraction (XRD), identifying the bottlenecks in the existing piezoelectric-metal stack. Further improvement of ScAlN/AlN-based mmWave acoustic resonators calls for better crystalline quality from improved thin-film deposition methods. [2023-0151]